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[ejs]

If "ifconfig ppp0" gives the MTU as 1500 for a PPPoA ADSL connection, I guess it's just not including the PPP overheads there, the same way as the MTU on eth0 shown by ifconfig doesn't include the Ethernet header.

[Charles]

merlin's version of asuswrt now has 1500mtu pppoe support built in

OpenWRT also has support here.

...its managing to fit 1500 bytes plus PPP overhead into 1500 bytes.

It's not, it fits into 1508 bytes with baby jumbo frames.

[boost]

It still makes no sense how its managing to fit 1500 bytes plus PPP overhead into 1500 bytes.  Its sorcery I tells ya! 

It's going to need the ISP network to be configured for >1500 bytes, I would guess. It sounds like a good idea if it is wholly supported by the provider.

I can imagine mega issues if support is sketchy, which is what appears to be the case here? Haven't you had speed issues, arguably worse than anyone else, for quite some time now, Chrysalis?

[Chrysalis]

boost, no tommy has it the worst and I have been on a 1492 mtu for at least 2-3 months now.

[Alex Atkin UK]

It's not, it fits into 1508 bytes with baby jumbo frames.

I understand how it could work on all-in-one devices, but how does that work via the HG612 when THAT has all its interfaces set to 1500?

As for OpenWRT, its a bummer its only available on fairly new builds as I can't get anything newer to boot on the PC I use as a router.

[iMx]

OpenBSD also has support as well

Code: [Select]

# ifconfig pppoe
pppoe0: flags=8851<UP,POINTOPOINT,RUNNING,SIMPLEX,MULTICAST> rdomain 11 mtu 1500
        priority: 0
        dev: vlan11 state: session
        sid: 0x226 PADI retries: 15 PADR retries: 0 time: 20d 15:43:02
        sppp: phase network authproto chap authname "bthomehub@btbroadband.com"
        groups: pppoe
        status: active
        inet 86.141.x.x --> 172.16.x.x netmask 0xffffffff
pppoe1: flags=8851<UP,POINTOPOINT,RUNNING,SIMPLEX,MULTICAST> rdomain 12 mtu 1500
        priority: 0
        dev: vlan12 state: session
        sid: 0x51 PADI retries: 16 PADR retries: 0 time: 20d 14:40:12
        sppp: phase network authproto chap authname "bthomehub@btbroadband.com"
        groups: pppoe
        status: active
        inet 86.141.x.x --> 172.16.x.x netmask 0xffffffff

[Alex Atkin UK]

The question is, does it improve performance?

[iMx]

I disagree that that is the main question/benefit - although most around here seem to favour raw speed over everything else  

For me, the main benefit is not having to scrub traffic on the firewall for a lower MSS and so reduces the load on the router/firewall, reduces potential CPU latency, and is generally 'cleaner'.

[Alex Atkin UK]

Not sure what you mean by scrubbed, as isn't MTU negotiated via TCP/IP between the client and server?

If that fails, your ISP will do the majority of the work there as downstream packets too large will be dropped at the other side of the PPPoE link due to not being able to fit down it.

So you are only dealing with traffic coming from your LAN.  All I did was set a 1480 MTU for my whole LAN and be done with it.  Although I noticed no difference whatsoever between that or when I had it to set 1500, as TCP/IP must have been doing it job.

And yes, I can still reach Gigabit speeds with an MTU of 1480.

I'm not saying you are wrong, I do not know that much when it comes to networking, but I haven't noticed any obvious problems using 1480.  Its why I wish I could try 1500, to see if it makes any difference at all.

[ejs]

Routers will typically have a firewall rule which modifies the MSS value in outgoing TCP SYN packets, replacing a too large value with the maximum allowed for the MTU. TCP MSS being the MTU minus the size of the TCP and IP headers.

[Chrysalis]

any mtu that isnt the standard 1500 means routers have to do more work, imx is correct.

[boost]

[AArdvark]

Boost,
That post should be a 'Sticky' quick OSI 7 layer to mtu explanation.
Nicely done.

Question:
What is the real impact of reducing your mtu ?

Also many people use Jumbo frames, where does this fit in ?
(Add this to the post perhaps?)

TIA

Send from LG G3 via Tapatalk (Typos & bad formatting are free)

[loonylion]

Also many people use Jumbo frames, where does this fit in ?
(Add this to the post perhaps?)

TIA

Send from LG G3 via Tapatalk (Typos & bad formatting are free)

Jumbo frames allows the use of an MTU of 7000 or 9000 vs the usual 1500 when on a gigabit ethernet network. it increases throughput for large files, but all devices in use must support the same version of it (i.e if some devices support 9kB jumbo frames and some only support 7kB jumbo frames, they should all be set to 7kB), and if any devices on the network do not support it, then enabling it will reduce performance rather than increase it due to the fragmentation explained above.

[ejs]

BT SIN 495 says section 3 of SIN 472 is not applicable to FTTC.

BT SIN 498 gives the maximum supported Ethernet frame size as 1530 bytes (not counting the VLAN tags).

Your computer might have discovered the MTU is lower than 1500, and if not, for TCP, as I said, many routers will modify the outgoing TCP SYN packets sent to open a connection, and replace the MSS with the appropriate lower value. So although a packet with the MSS set to 1460 leaves your computer, the router changes the MSS in the packet to 1452, which the web server receives, and the web server replies acknowledging the MSS of 1452.

With an integrated VDSL2 modem+router, transporting 1500 byte IP packets should be fairly straightforward because the PPPoE layer is added within the router and then carried directly over the VDSL2 link (which operates in PTM mode).

With a separate VDSL2 modem and router, the PPPoE packets from the router have to go over the Ethernet link between the router and the modem. To fit into standard sized Ethernet frames, to allow for the PPPoE headers, that leaves 1492 bytes for the IP packets (IP header + TCP header + data). Or you need jumbo frames over this Ethernet link between the modem and router to enable it to carry slightly oversized Ethernet frames.

I don't really want to think about the PPPoEoA of using PPPoE over ADSL even though the ADSL layer will still be carrying ATM cells, it's not directly relevant to FTTC.